polynucleotide kinase

Hello:
I am trying to do polynucleotide kinase exchange reactions on RNA. In the
protocol described in Maniatis and in Current Protocols, exchange reactions are
distinguished from the 'forward reaction' by the use of imidazole buffer
instead of TRIS. Does anyone know why, an how critical it is to use this
buffer system. Please send replies to KIM at FLOVAX.LANL.GOV
Thanks
Daniel Kim